Double-base-connected bipolar transistors with passive components preventing accidental turn-on

ABSTRACT

The present application discloses new approaches to providing “passive-off” protection for a B-TRAN-like device. Even if the control circuitry is inactive, AC coupling uses transient voltage on the external terminals to prevent forward biasing an emitter junction. Preferably the same switches which implement diode-mode and pre-turnoff operation are used as part of the passive-off circuit operation.

CROSS-REFERENCE

Priority is claimed from U.S. provisional application 62/308,660, whichis hereby incorporated by reference.

BACKGROUND

The present application relates to solid-state switches using bipolarconduction, and more particularly to bipolar transistors using two baseconnections.

Note that the points discussed below may reflect the hindsight gainedfrom the disclosed inventions, and are not necessarily admitted to beprior art.

Published US application US 2014-0375287 (which is hereby incorporatedby reference in its entirety) discloses (inter alia) novel bidirectionalbipolar transistors known as “B-TRANs.” Further improvements to theB-TRAN device and its modes of operation where disclosed in applicationSer. Nos. 14/937,814 and 14/882,316.

One sample embodiment of a B-TRAN can be seen in FIG. 1B. Note that thetwo surfaces of the die are essentially identical.

A sample circuit symbol is shown in FIG. 2. This circuit symbolresembles that of a bipolar junction transistor, except that two baseconnections are shown. This corresponds to the device structure of FIG.1B, where two different base contact regions are placed on the twosurfaces of the die.

FIG. 3 shows one sample embodiment of a B-TRAN drive circuit, asextensively described in the parent applications.

FIG. 4 shows another sample embodiment of a B-TRAN. In this embodimentthe trenches contain field plates; the capacitive coupling to the fieldplates helps to smooth the nearby voltage gradient in the verticaldirection.

The preferred modes of operation of the BTRAN are surprisingly complex.To achieve high bipolar gain reliably, in a bidirectional device, theparent applications teach that the following stages of operation can beused.

At turn-on, an initial flow of current is allowed to occur in “diodemode” before bipolar transistor operation begins. In diode mode, thevoltage drop across the device is (of course) at least a diode drop; butwhen base current drive is applied, the forward voltage drop can bereduced to a few hundred millivolts.

At turn-off, base current is disabled first, so that the device is againoperating as a diode. After this, the device can be put into the “activeoff” mode, where one of the two junctions is reverse biased and blockscurrent.

A further surprising mode taught in the parent application is the“passive-off” mode. A problem with a fully bidirectional device is thatthe bipolar gain can interfere with current blocking in the off state.To avoid this, the emitter junction on either surface of the device isclamped to avoid any significant forward bias. (Properly, the “emitterjunction” referred to here is the junction between either of the(typically n-type) emitter/collector regions and the (typically p-type)substrate.) By keeping the emitter junctions well away from turn-on,minority carrier injection is limited, and the gain of the bipolartransistor does not degrade the breakdown voltage.

Double-Base-Connected Bipolar Transistors with Passive ComponentsPreventing Accidental Turn-on

The present application discloses new approaches to providing“passive-off” protection for a B-TRAN-like device. Even if the controlcircuitry is inactive, AC coupling uses transient voltage on theexternal terminals to prevent forward biasing an emitter junction. Thusthe transistor's gain is automatically prevented from degrading thebreakdown voltage when the device is off. Preferably each surface of thedevice has a base contact region and an emitter/collector region; thepolarity of the externally applied voltage will determine which of thetwo emitter/collector regions will act as emitter, and which ascollector. The passive turnoff circuit clamps each base contact regionto less than a diode drop from the neighboring emitter/collector region,so that bipolar transistor operation is avoided.

The new passive-off circuitry is particularly (but not only)advantageous in soft-switched applications, such aspower-packet-switching (PPSA) converter.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments and whichare incorporated in the specification hereof by reference, wherein:

FIG. 1A shows an example of a B-TRAN switching circuit, including“passive-off” circuitry which avoids breakdown voltage degradation dueto amplification of transients.

FIG. 1B shows one sample embodiment of a B-TRAN.

FIG. 2 shows a sample circuit symbol for a B-TRAN.

FIG. 3 shows one sample embodiment of a B-TRAN drive circuit.

FIG. 4 shows another sample embodiment of a B-TRAN.

FIG. 5 shows an alternative circuit realization of the passive-offfunction.

DETAILED DESCRIPTION OF SAMPLE EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to presently preferred embodiments(by way of example, and not of limitation). The present applicationdescribes several inventions, and none of the statements below should betaken as limiting the claims generally.

The present application describes a new implementation of thepassive-off mode described in published US application US 2014-0375287.

The present application discloses new approaches to providing“passive-off” protection for a B-TRAN-like device. Even if the controlcircuitry is inactive, AC coupling uses transient voltage on theexternal terminals to prevent forward biasing an emitter junction.

The exemplary circuit of FIG. 1A shorts the e-base to emitter whenexternal voltage is applied across collector to emitter (Vce), andbefore base control power is available, so as to allow the B-TRAN toblock the applied Vce.

The new passive-off circuitry is particularly applicable tosoft-switched applications, such as power-packet-switching (PPSA)converter. For hard-switched applications, the capacitor C1 will retardthe switching speed. A voltage-limiting circuit with solid-stateswitches (or even with mechanical relays) may be more advantageous insuch applications.

Device 101 is a “B-TRAN” type transistor, i.e. a bipolar transistor withtwo base contact regions which are separately operated. This is a fullybidirectional device, with voltage blocking and current conductionavailable in both directions. In this example the B-TRAN 101 is assumedto be an npn device, i.e. the emitter/collector regions are n-type, thesubstrate is p-type, and the base contact regions are p-type. (Otherdevice structures may be present too.)

In this example, assume that the external terminal at the top of thepage starts to go high, and the control circuitry is not active. Thiswill make connection EC2 the collector terminal, and EC1 the emitterterminal. Connection B1 therefore operates as the e-base, and connectionB2 as the c-base.

Some charge will necessarily shift to create a depletion region, whichis required to block conduction. However, the challenge is to avoid anygain on the emitter side, which would degrade the breakdown voltage. Thevoltage at the emitter junction (EC1/B1) must therefore be kept below adiode drop.

Resistor R1 holds capacitor C1 at zero voltage when the device is fullypowered down. When terminal EC2 starts to go positive, current will flowthrough C1 to pull the gates of MOS transistors Q5 and Q6 high. Thisturns them on, to pull the e-base terminal (B1 at the moment) toward theemitter (EC1 at the moment).

Note that the MOS transistors Q3+Q4 and Q5+Q6, which were used fordiode-mode and pre-turnoff modes, are now also used as part of thepassive-off circuitry. This is significantly different from thepassive-off circuitry disclosed in previous applications regardingoperation of a B-TRAN device.

Note also that resistor R2 connects the gates of p-channel MOStransistors Q1 and Q2 together. This subcircuit helps to keeptransistors Q3 and Q4 off while Q5 and Q6 are turning on.

The present application describes a new implementation of thepassive-off mode described in published US application US 2014-0375287.

FIG. 5 shows an alternative circuit realization of the passive-offfunction. The normally-ON switches (implemented as JFETs in FIG. 3,which corresponds to FIG. 12 of WO2014/210072) are replaced byresistors.

In selecting the resistors to replace the normally-ON switches, the ON-and OFF-state behaviors must be balanced. The breakdown voltage inpassive-off mode must be sufficiently high, while not significantlyreducing the gain.

A B-TRAN is in the “active off-state” when the e-base (base on emitterside) is shorted to the emitter, and the c-base (base on the collectorside) is open. In this state with the NPN B-TRAN, the collector is theanode (high voltage side), and the emitter is the cathode (low voltageside).

The B-TRAN is also off when both bases are open, but due to the highgain of the B-TRAN in this state, the breakdown voltage is low. Theseries combination of a normally-on JFET (as in earlier versions, shownin FIG. 3) and a Schottky diode, or a resistor (as taught herein, shownin FIG. 5) and a Schottky diode, attached between each base on itsrespective emitter/collector, as previously disclosed, willsignificantly increase the blocking voltage in this “passive off-state”.The JFETs, if present, are turned off during normal operation.

One advantageous sample method for turn-on is to simultaneously, fromthe active off-state and blocking forward voltage, open the e-base toemitter short while shorting the c-base to the collector. Thisimmediately introduces charge carries into the highest field region ofthe depletion zone around the collector/base junction, so as to achievevery fast, forward biased turn-on for hard switching, very similar toIGBT turn-on.

Another advantageous sample turn-on method, from the active off-state,is to have the circuit containing the B-TRAN reverse the B-TRANpolarity, which produces the same base state described in the hardturn-on method, but at near zero voltage. That is, the e-base which isshorted to the emitter becomes the c-base shorted to the collector asthe B-TRAN voltage reverses from the active off-state polarity. Andagain, turn-on is fast.

In a third sample turn-on method from the active off-state, the e-baseis disconnected from the emitter, and connected to a current or voltagesource of sufficient voltage to inject charge carriers into the baseregion. This method is likely slower, since the charge carriers go intothe base just below the depletion zone. Also, it is known that carrierinjection into the e-base results in inferior gain relative to carrierinjection into the c-base.

After turn-on is achieved with either of the methods using the c-base,Vce is more than a diode drop. To drive Vce below a diode drop, turn-ongoes to the second stage of increased charge injection into the c-basevia a voltage or current source. The amount of increased chargeinjection determines how much Vce is reduced below a diode drop.Injection into the e-base will also reduce Vce, but the gain is muchlower than with c-base injection.

Turn-off can be achieved by any of several methods. The mostadvantageous method is a two-step process. In the first step, the c-baseis disconnected from the carrier injection power supply and shorted tothe collector, while the previously open e-base is shorted to theemitter. This results in a large current flow between each base and itsemitter/collector, which rapidly removes charge carriers from the driftregion. This in turn results in a rising Vce as the resistivity of thedrift region increases. At some optimum time after the bases areshorted, the connection between the c-base and the collector is opened,after which Vce increases rapidly as the depletion region forms aroundthe collector/base junction.

Alternately, turn-off can be achieved by simply opening the c-base andshorting the e-base to the emitter, but this will result in higherturn-off losses since the drift region (base) will have a high level ofcharge carriers at the start of depletion zone formation.

Or, turn-off can be achieved by simply opening the c-base and leavingthe e-base open, but this will result in the highest turn-off losses andalso a low breakdown voltage.

Advantages

The disclosed innovations, in various embodiments, provide one or moreof at least the following advantages. However, not all of theseadvantages result from every one of the innovations disclosed, and thislist of advantages does not limit the various claimed inventions.

-   -   Improved efficiency in power conversion systems;    -   Power semiconductor devices with more ruggedness;    -   Power semiconductor devices with higher breakdown voltage;    -   Power semiconductor devices with lower on-resistance:    -   Power semiconductor devices with lower cost.

According to some but not necessarily all embodiments, there isprovided: The present application discloses new approaches to providing“passive-off” protection for a B-TRAN-like device. Even if the controlcircuitry is inactive, AC coupling uses transient voltage on theexternal terminals to prevent forward biasing an emitter junction.Preferably the same switches which implement diode-mode and pre-turnoffoperation are used as part of the passive-off circuit operation.

According to some but not necessarily all embodiments, there isprovided: A switching circuit, comprising: a power semiconductor devicewhich includes first and second first-conductivity-typeemitter/collector regions on respective surfaces of asecond-conductivity-type semiconductor die, separately defining firstand second emitter junctions, and a first base contact region on thesame surface as the first emitter/collector region, and a second basecontact region on the same surface as the second emitter/collectorregion, both base contact regions separately making ohmic contact to thesemiconductor die; first drive transistors which, when ON, operativelyconnect the first base contact region to the first emitter/collector;and second drive transistors which, when ON, operatively connect thesecond base contact region to the second emitter/collector region; and atransient coupling circuit which is operatively connected toelectrically connect voltage slew across the emitter/collector regionsto activate the drive transistors of whichever of the emitter/collectorregions is becoming the emitter, to limit the voltage between thatemitter/collector region and its corresponding base contact region; acontrol circuit which drives the first and second base contact regionsindependently, to control turn on and turn off of conduction, includingswitching phases where the first drive transistors and second drivetransistors connect at least one of the emitter/collector regions withits corresponding base contact region; whereby forward voltage on thefirst emitter junction is limited to less than the forward diode voltagedrop characteristic of the first emitter junction, and forward voltageon the second emitter junction to less than the forward diode voltagedrop characteristic of the second emitter junction; and thereby leakagecurrents are not amplified when the control circuit is inactive, andbreakdown voltage is not degraded by amplification of leakage currents.

According to some but not necessarily all embodiments, there isprovided: A switching circuit, comprising: a power semiconductor devicewhich includes first and second first-conductivity-typeemitter/collector regions on respective surfaces of asecond-conductivity-type semiconductor die, separately defining firstand second emitter junctions, and first and second base contact regionson the same two surfaces as the first and second emitter/collectorregions respectively, both base contact regions separately making ohmiccontact to the semiconductor die; first drive transistors which, whenON, operatively connect the first base contact region to the firstemitter/collector; and second drive transistors which, when ON,operatively connect the second base contact region to the secondemitter/collector region; and a transient coupling circuit whichelectrically couples voltage slew from a collector-side one of theemitter/collector regions to activate the drive transistors of theopposite emitter/collector; a control circuit which drives the first andsecond base contact regions independently, including switching phaseswhere the first drive transistors and/or second drive transistorsconnect at least one of the emitter/collector regions with itscorresponding base contact region; whereby forward voltage on the firstemitter junction is limited to less than the forward diode voltage dropcharacteristic of the first emitter junction, and forward voltage on thesecond emitter junction to less than the forward diode voltage dropcharacteristic of the second emitter junction.

MODIFICATIONS AND VARIATIONS

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given. It is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

None of the description in the present application should be read asimplying that any particular element, step, or function is an essentialelement which must be included in the claim scope: THE SCOPE OF PATENTEDSUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none ofthese claims are intended to invoke paragraph six of 35 USC section 112unless the exact words “means for” are followed by a participle.

The claims as filed are intended to be as comprehensive as possible, andNO subject matter is intentionally relinquished, dedicated, orabandoned.

What is claimed is, among others (and, without exclusion, in addition toany other points which are indicated herein as inventive and/orsurprising and/or advantageous):
 1. A switching circuit, comprising: apower semiconductor device which includes first and secondfirst-conductivity-type emitter/collector regions on respective surfacesof a second-conductivity-type semiconductor die, separately definingfirst and second emitter junctions, and a first base contact region onthe same surface as the first emitter/collector region, and a secondbase contact region on the same surface as the second emitter/collectorregion, both base contact regions separately making ohmic contact to thesemiconductor die; first drive transistors which, when ON, operativelyconnect the first base contact region to the first emitter/collector;and second drive transistors which, when ON, operatively connect thesecond base contact region to the second emitter/collector region; and atransient coupling circuit, including at least one series resistor,which is operatively connected to electrically connect voltage slewacross the emitter/collector regions to activate the drive transistorsof whichever of the emitter/collector regions is becoming the emitter,to limit the voltage between that emitter/collector region and itscorresponding base contact region; a control circuit which drives thefirst and second base contact regions independently, to control turn onand turn off of conduction, including switching phases where the firstdrive transistors and second drive transistors connect at least one ofthe emitter/collector regions with its corresponding base contactregion; whereby forward voltage on the first emitter junction is limitedto less than the forward diode voltage drop characteristic of the firstemitter junction, and forward voltage on the second emitter junction toless than the forward diode voltage drop characteristic of the secondemitter junction; and thereby leakage currents are not amplified whenthe control circuit is inactive, and breakdown voltage is not degradedby amplification of leakage currents.
 2. The switching circuit of claim1, wherein the semiconductor die is silicon.
 3. The switching circuit ofclaim 1, wherein the emitter/collector region on the first said surfaceis not electrically connected to the emitter/collector region on thesecond said surface, except through the semiconductor die itself.
 4. Theswitching circuit of claim 1, wherein the base contact region on thefirst said surface is not electrically connected to the base contactregion on the second said surface, except through the semiconductor dieitself.
 5. The switching circuit of claim 1, wherein the first andsecond first-conductivity-type emitter/collector regions are bothn-type.
 6. The switching circuit of claim 1, wherein thesecond-conductivity-type semiconductor die is p-type.
 7. The switchingcircuit of claim 1, wherein the first and second base contact regionsare both p-type.
 8. The switching circuit of claim 1, wherein the firstand second first-conductivity-type emitter/collector regions are bothn-type, and wherein the second-conductivity-type semiconductor die andthe first and second base contact regions are all p-type.
 9. Theswitching circuit of claim 1, wherein the transient coupling circuitdoes not include any series active devices.